A circuit known as a flyback converter is a switch mode power supply circuit commonly used in applications such as AC-to-DC wall adapter power supplies and battery chargers.
FIG. 1 shows a block diagram of a conventional flyback converter 100 that is controlled from a primary side and that operates in an emitter switching configuration. Flyback converter 100 includes a transformer 102 that has primary, secondary, and auxiliary windings. On the primary side, flyback converter 100 includes resistor 104, capacitor 106, switcher circuit 108, transistor switch 110, and diode 112. On the secondary side, flyback converter 100 includes a secondary rectifier 114, a bleed resistor 116, and an output capacitor 118. An alternating current (AC) line voltage may, for example, be rectified by a full wave bridge rectifier (not shown) and an associated smoothing capacitor (not shown) so that the rectified and smoothed rough DC voltage is present at terminal Vin.
Flyback converter 100 operates by repeatedly closing and opening the switch 110. Closing the switch 110 causes a current to flow from the terminal Vin, through the primary side windings (having Np number of windings) of the transformer 102, through the switch 110, and into the SW terminal of the switcher circuit 108. Within the switcher circuit 108, the current is routed to flow out of a current sense (CS) terminal. Opening the switch 110 causes voltage at the dot end of the secondary side windings (having Ns number of windings) to swing positive, which causes current flow to the output capacitor to produce an output voltage at terminal Vout. In one example, a rough DC voltage is present on the secondary side of transformer 102 at output voltage terminal Vout. The switcher circuit 108 controls the closing and opening of switch 110 during operation.
The start-up (or power-up) time of the flyback converter 100 is a measure of the time it takes to obtain a desired regulated output voltage at the terminal Vout. During start-up, the voltage at terminal Vout rises from zero (0) volts to the desired regulated output voltage. The start-up time depends on the RC time constant of the system. The RC time constant is the product of the resistance of resistor 104 and the capacitance of capacitor 106. Resistor 104 and capacitor 106 provide the initial start-up energy for switcher circuit 108. Once the Flyback converter is stable, switcher circuit 108 is powered by the auxiliary side winding (with Na number of turns) of transformer 102 via rectifier 112.
Standby mode occurs when a rectified input voltage is present at terminal Vin but no device draws power from terminal Vout. Power consumption during standby mode depends inversely on the resistance of resistor 104. Capacitor 106 is used to start-up the flyback converter 100 by supplying enough current to the base terminal of transistor 110 to switch current through the transformer and also to provide a bias voltage to switcher circuit 108 prior to the auxiliary winding supplying power to switcher circuit 108. Large values of resistance of resistor 104 and capacitance of capacitor 106 result in a large RC time constant and a long start-up time. However, smaller values of the resistor 104 result in larger standby power utilization.
FIG. 2 shows a graph 200 that illustrates an approximate voltage waveform at terminal VDD during start-up. In the graph 200, the voltage at terminal VDD rises slowly based on the RC time constant until a turn-on threshold is reached at time T1. For example, the start-up time may be approximately 2.5 seconds. Unfortunately, in some applications, this long start-up time is not acceptable. If the start-up time is decreased by decreasing the resistor 104, then the standby power utilization will correspondingly increase, which may be undesirable.
Therefore, it is desirable to have a switching regulator that provides fast start-up times and low standby power utilization.